Titanium is a very important metal for many industrial applications, because of its combination of high strength and relatively low weight. Titanium-based alloys are therefore the material of choice for high performance components, such as compressor discs for aircraft propulsion systems. A wide range of alloys are available, each conferring a particular combination of characteristics to the component.
Titanium is usually obtained from various ores, such as ilmenite, rutile (TiO.sub.2), and titanate. Several commercial methods for extracting the metal from the ore are well-known. One general technique involves the reduction of titanium tetrachloride with sodium (the Hunter process) or with magnesium (the Kroll process). Since titanium is highly reactive with oxygen, nitrogen and hydrogen, these processes are usually carried out in vacuum, or in an inert atmosphere like helium or argon. The titanium precipitates as a spongy mass, and can be consolidated by re-melting.
Since titanium is used in alloys intended for critical applications, it is quite clear that the titanium sponge itself must be free of components which would detract from its quality. For example, nitrogen is sometimes present in the alloy in the form of nitrogen-rich "inclusions". These inclusions can shorten the fatigue life of titanium-based alloys, rendering the materials susceptible to failure--especially under high temperature use. Thus, eliminating or reducing the presence of nitrogen is very critical in titanium processes.
Those of skill in the art of titanium refining recognize that the presence of impurities like nitrogen change the color of the titanium sponge (the element itself is usually silvery-white in pure form). Titanium sponge fragments with a desirably low amount of nitrogen, e.g., less than about 1.0 wt. %, usually have a silver or dull gray surface color. Titanium fragments with higher amounts of nitrogen have different colors. For example, fragments with a nitrogen content above about 18.4 wt. % often have a bright yellow color.
These important color distinctions allow the titanium sponge fragments to be separated after being precipitated. Typically, the fragments are passed on a conveyor belt of some sort, while individuals observe the fragments and manually discard those that have colors characteristic of high nitrogen content. (Fragments are discarded for other reasons as well, e.g., if they contain magnesium chloride inclusions that are visible to the eye).
The process of having individuals visually review sponge fragments for color deviation can be quite time-consuming. It can also be labor-intensive if greater amounts of fragments need to be processed, i.e., if the conveyor belt speed needs to be increased, or if multiple conveyor belts are needed.
It should thus be apparent that new techniques for sorting titanium sponge fragments by color would be welcome in the art. These techniques should permit high-speed sorting, with a high level of accuracy. They should also eliminate or minimize the occurrence of human error in the sorting process. Moreover, the techniques should be readily adaptable to a variety of production lines used in ore-processing industries.